How is LED tested in order to determine its durability?

The durability of LED does not depend on the operating cycle and the number of on/​off inclusions. Carrying study on LED durability is problematic because of its long lighting time. Even when working 24 hours a day to test the LED during 50 thousand hours would take 57 years, results obtained after the completion of the study would be less useful because of changes in LED construction, which would occur during this time. Because of the facts mentioed above, nowadays testing the durability of LED or LED modules take place in 2 stages:

  • Stage 1LED or LED module is being tested at a current and/​or the maximum voltage during 1000 hours (heating and stabilization period). It is crucial as for the majority of LEDs, luminous flux increases during the first 1000 hours of operation.
  • Stage 2 – turned LED works for the following 5000 hours. Value of luminous flux measured after the heating period (1000 operating hours) is accepted as the initial value (referential). These measurements made between 1000 and 6000 hours of operation are comparable with the initial level (1000 hours). During those 6000 hours the luminous flux will not fall below 70% value of the initial lumious flux (applicable for the general lighting), an exploitation is made on the basis of obtained data. Far exploitation may be performed only for the period of time which is 6 times testing time, or 36000 hours.

Due to this method, verification of declared long durability is possible. Another option to indicate the durability is currently being discussed within the IEC1. It introduces an evaluation of LED durability on the basis of luminous flux measurements during the operation at three temperatures: 550C, 850C and temperature specified by the manufacturer. The temperature is measured at the measuring point on the LED module, which is adviseable by the manufacturer. Luminous flux measurements are made for 20 LED items for at least 6000 hours.

The expected LED durability is extrapolated on the basis of executed measurements of luminous flux drop, under the condition that the luminous flux is lower than 30% of the initial value. The basis for this method is strict dependence of LED durability (luminous flux drop) on the temperature obtained in the connector.